Digital camera devices use a microelectronic imaging sensor (image sensor) to capture digital images of a scene. The camera devices are often integrated into portable or battery-operated devices such as dedicated video or still cameras, cellular phones and multi-function or smart phones.
For improved image capture quality, some digital camera devices use the more traditional, electro-mechanical shutter mechanism, rather than a purely electronic shutter, for controlling exposure of the image sensor. In an electro-mechanical shutter, an actuator mechanism often referred to as a voice coil motor is coupled to move one or more blades, in response to being energized by an input drive current. When the motor is not energized, the blades can be biased into a shutter open or a shutter closed state, using for example a small spring.
Electro-mechanical shutters are used in digital cameras in the following manner. In one instance, the shutter blades are biased open by the spring, when the camera is first enabled or powered-on. This is referred to as a “normally open” state, which allows video of the scene to be captured by the image sensor and displayed to the user, during a preview or viewfinder mode of operation. Next, when the user actuates the shutter release button to take a picture (also referred to as an “exposure”), the image sensor is electrically reset to start an integration interval during which the sensor responds to light from the scene to form the captured image. At the end of the integration interval, the motor is automatically energized, by raising its drive current. The motor responds by moving the blades to a shutter closed state. This stops the sensor from further responding to light from the scene. At this point, the electrical signals representing the captured image are read from the sensor (referred to as a readout operation interval) and stored in memory, for subsequent processing and/or viewing. Upon completion of readout, the motor is then de-energized by lowering the drive current to essentially zero. This allows the spring to bias the blades back into the shutter open condition, which allows the viewfinder mode to resume (letting the user prepare for taking the next picture).
The above-described shutter mechanism is referred to as a unipolar shutter, where the drive current is raised and then maintained at a constant level for essentially the entire duration of readout, in only one direction. In this case, the drive current is raised to close the blades, and then returned to zero, at which point the soft spring urges the blades to their open state.
There are situations where the blades can become stuck in the closed or in an almost closed state when the motor has been de-energized, due to electrostatic forces, which the soft spring cannot overcome. To avoid this problem, a bipolar shutter mechanism has been suggested. In that case, the motor is also operated in reverse, i.e. its drive current is “raised” in the opposite direction. The motor responds to this by moving the blades in the reverse direction, in this example to the shutter open state.
With increasingly greater functionality being integrated into portable devices, the design of the constituent components or functions of the device is typically carefully scrutinized, to ensure that power consumption is reduced whenever possible. The digital camera device is no exception. Thus, in the case of the bipolar shutter, to maintain low power consumption, the drive current is pulsed in both directions (rather than being kept “on” for the entire duration of a readout interval or an integration interval). An additional mechanism, such as one that uses a soft spring, is provided that helps maintain the blades in their open or closed states when the motor is de-energized. This helps lower the overall power consumption of the camera device, because the motor's drive current is only turned on for a brief interval at the end of each exposure and at the start of the next exposure.